dt-bindings: Add vendor prefix for Motorola

Motorola was involved in semiconductor and mobile phone business.
The "motorola," prefix is already used by a couple of bindings:

* rtc/rtc-cmos.txt
* mfd/motorola-cpcap.txt
* regulator/cpcap-regulator.txt

Apart from that it is used in the DT file for the Droid 4 mobile
phone.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sebastian Reichel <sre@kernel.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>